Integrated Distill library

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #20
| License       | MIT
| Doc PR        | -

The installer now integrates the [Distill library](https://github.com/raulfraile/distill) to download the best format (ZIP or TGZ) and to uncompress it using the native operating system commands:

![symfony-installer](https://cloud.githubusercontent.com/assets/73419/5088468/330ba72c-6f34-11e4-9fdc-89e462dd296b.gif)
Are the two spaces in front of "Symfony" intended?